The strategy of the proposed research is to use electron microscopic tomography(EMT), video light microscopy and 3-D immuno-electron microscopy to evaluate several current hypothesis concerning chromosome motion and kinetochore/centromere complex (KCC) organization.The latter include: models of the interaction between kinetochore microtubules (kMT's) and the KCC during movement toward and away from the spindle pole; how the direction of motion is controlled; if the morphological changes associated with the dynamic instability of MTs in vitro can be detected in kMTs; the repeat subunit model for the kinetochore outer plate organization; and the possible role of specific DNA sequences in KCC function. The results of these investigations will significantly improve our understanding of how mitosis works, and lay the foundation for future studies that identify the chemical mechanisims responsible for many of the various chromosome motions during mitosis.